1. Field of the Invention
The present invention relates generally to the field of global positioning systems. More specifically, the present invention relates to components and methods useful for reducing power consumption in global positioning system receivers and improving processes for locating and tracking assets.
2. Description of the Related Art
The Global Positioning System (GPS) is used to, among other functions, determine location. Developed initially by the United States Department of Defense, the GPS is comprised of space vehicles or satellites (hereafter “SV”), a Master control facility and one or more receivers, usually referred to as GPS receivers. More particularly, there are 31 SVs that orbit the earth in approximately 12 hours. The SV orbits repeat virtually the same ground track once each day and repeat the same track and configuration over any point with reliable regularity. There are six orbital planes, several SVs per plane, equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. Accordingly, a user located anywhere on the Earth will be visible to between five and eight SVs.
The Master Control facility (MC facility) is located at Schriever Air Force Base in Colorado. The MC facility monitors the SVs and measures signals from the SVs which are then incorporated into orbital models for each SV. Based on orbital models, precise orbital data (ephemeris) and SV clock corrections are computed for each SV and uploaded to the SVs. The SVs in turn transmit subsets of the orbital ephemeris data as part of a Navigation Message to GPS receivers using radio signals. The Navigation Message contains ephemeris and other parameters needed to compute a position estimate (receiver position and local clock error). A Navigation Message is formatted as shown in FIG. 1. The each SV transmits a Coarse/Acquisition (C/A) signal on the L1 frequency (1575.42 MHz). A Navigation Message continuously bi-phase modulates the L1 carrier at 50 bits/sec. This signal is then spread by a Gold code unique to each SV. The C/A code chip rate is 1.023 MHz. With 1023 chips per code epoch, the duration of a single epoch is exactly 1 ms. Thus there are 20 epochs per bit. All SVs transmit synchronously.
Referring to FIG. 1, The Navigation Message is a continuous 50 bits/second data stream modulated onto the carrier signal of every SV. It is a telemetry message, and the data is transmitted in logical units called frames. For GPS a frame is 1500 bits long, so it takes 30 seconds to be transmitted. Every SV begins to transmit a frame precisely on the minute and half minute, according to its own clock. Each frame is divided into five subframes, each subframe is 300 bits long. Subframes 1, 2 and 3 contain the high accuracy ephemeris and clock offset data. The content of these subframes 1, 2 and 3 is the same for a given satellite for consecutive frames for periods lasting as long as two hours. Subframe 1 contains second degree polynomial coefficients used to calculate the satellite clock offset. Subframes 2 and 3 contain orbital parameters. Subframes 4 and 5 are subcommutated, which means that consecutive subframes have different content. This data does repeat, but 25 consecutive frames of subframe 4 and 5 data must be collected before the receiver has all of the unique data being transmitted by the SV. An SV transmits the same data in subframes 4 and 5 until it is next uploaded, or usually for about 24 hours. Subframes 4 and 5 contain the almanac data and some related health and configuration data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.
Each subframe is divided into 10 words of 30 bits each. Words 1 and 2 have the same format in every subframe. Word 1 is called the telemetry word. The first 8 bits in the telemetry word contain a sync pattern, used by the GPS receiver to help synchronize itself with the Navigation Message and thus be able to correctly decode the data. Word 2 contains the truncated Z-count. This is the time according to the SV's clock when the end of the subframe will be transmitted, with a scale factor of 6 seconds.
The MC facility uploads data to the SVs at approximately 24 hour intervals, sending the SV all of the data that the SV will transmit during the next 24 hours, plus data for extra days in case an upload is unexpectedly delayed. An upload contains roughly 16 subframe's worth of 1, 2 and 3 subframe data. An SV may begin transmitting new data, referred to as a cutover, after an upload at any time of the hour, but subsequent transmissions only occur precisely on hour boundaries. Each subframe 1, 2 and 3 data set is transmitted for no more than two hours with some transmitted for exactly an hour, some for exactly two hours and some (either immediately before or after an upload) broadcast for a period of less than two hours.
Subframe 1 contains a clock offset time-of-applicability (this is the fit time for the polynomial), and either subframe 2 or 3 contains an ephemeris time-of-applicability. The two time-of-applicability values are almost always the same, and for a cutover that begins on an hour epoch, the time-of-applicability values are almost exactly two hours later than the initial transmission time of the subframe 1, 2 and 3 data sets. Each of the three subframes also contains an index value which allows the receiver to verify that the three subframes are part of the same data set.
FIG. 2 is a schematic for a typical GPS receiver. The GPS receiver attempts to track all 5 to 12 SVs within view simultaneously, receiving the combined signals transmitted from the SVs at the RF front end 10. The data is digitized at an A/D converter 20. The reference Gold codes 30 are cross-correlated with the received data 40 and the signal delay information 50 is converted to a range estimate, which before any corrections are applied, is called pseudorange (PR), and is input to the central processing unit (CPU) 70. The Navigation Message is demodulated 60 and ephemeris data from the Navigation Message allows calculation of SV position. The Navigation Message also contains parameters used to correct the PRs. Given corrected PRs and SV positions, the CPU computes a position estimate 80. Four PRs and the positions of the four corresponding SVs allows solution of four range equations in four unknowns: user position in an earth-fixed coordinate system (x,y,z) and user clock error. If more than four PRs are available, a least squares solution can produce better accuracy. With this information the GPS receiver can also calculate distance traveled, time traveled, speed of travel, average speed, travel history, estimated time of arrival, etc. for navigational purposes. The GPS can be used for precise positioning using GPS receivers at reference locations providing corrections and relative positioning data for remote receivers. Surveying, geodetic control, and plate tectonic studies employ this method. Time and frequency dissemination, based on the precise clocks on board the SVs and controlled by the monitor stations, is another use for GPS. Astronomical observatories, telecommunications facilities, and laboratory standards can be set to precise time signals or controlled to accurate frequencies by special purpose GPS receivers.
On power-up, a GPS receiver must read all or part of the Navigation Message before outputting its first position estimate. This time-to-first-fix can be tens of seconds to minutes, depending on the receiver's initial state.
The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the Navigation Message and the standard position service SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay, but more importantly it carries encrypted navigation data for military precise positioning service PPS equipped receivers.
Two binary signals modulate the L1 and/or L2 carrier phase. The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1.023 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates the L1 carrier signal, “spreading” the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 chips (one millisecond). There is a different C/A code PRN (Gold code) for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil SPS. The P-Code (Precise) modulates both the L1 carrier, in quadrature with the C/A code, as well as the L2 carrier. The P-Code is a very long (seven days) 10.23 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys. The P (Y)-Code is the basis for the PPS.